bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> > Yes, I wasn't clear enough, I should have written "when the file is 
> > large enough (say 1 GB), even without long lines".
>
> What kind of scenario are you thinking of that would exhibit this 
> slowness in a 1 GB file?
>
> If we're talking about syntax-ppss only, regular editing operations 
> (typing and deleting code) limited to, say, one screen should trigger 
> only a rescan of a limited area inside such buffer.

That's true, but with such big files, the initial scan is slow.  So the 
scenario is simple: you open a big enough file, type M->, and C-p.  M-> 
will be instantaneous, and C-p will take a while, because of syntax-ppss.